Hitherto, conventional polyoxymethylene resins have been used widely as engineering plastics for materials of parts in various mechanism portions and equipments for office automation, for the reasons that the polyoxymethylene resins possess well-balanced mechanical properties as well as excellent wear resistant characteristics.
However, only on the basis of their native wear resistance characteristics, conventional polyoxymethylene resins can hardly be said as suitable plastics to be used as materials of parts for sliding portions which should have physical properties good enough to satisfy the requirements. Particularly, in use of conventional polyoxymethylene resins at high temperature, the friction coefficient and wear resistance thereof are increased, thus the sliding characteristics of the resins become worse.
Therefore, at present, for the purpose to improve the sliding characteristics at high temperature, conventional polyoxymethylene resins are used frequently by coating grease to the sliding portions.
However, such a coating of grease prolongs the operation period in working steps. Furthermore, in case of using a fluorocarbon solvent in the coating step, then environmental pollution problems will occur. Under the circumstances, there have been eagerly expected the development of polyoxymethylene resin composition which can be used without need of coating grease.
Currently, however, there have not been known any specific technology for improving the sliding characteristics at high temperature of the polyoxymethylene resin.
As to prior art references related to improvement of sliding characteristics of polyoxymethylene resins, there have been known some literatures e.g., method of adding an ester of 1,4-butanediol with a fatty acid to polyoxymethylene resin (JP-B-55-23304); method of adding a boric acid ester of monofatty acid glyceride (JP-B-57-15616); method of adding polyolefin having active hydrogen atoms and polyethylene and an isocyanate (JP-A-4-126758); method of adding a copolymer of .alpha.-olefin with ethylene-vinyl monomer (JP-A-49-40346); method of adding powder of polyethylene having super high molecular weight (JP-A-60-144351); method of adding a polyether type copolymer having an average molecular weight over 500 (JP-A-5-70678); method of adding a thermoplastic elastomer and a lubricating agent so as to be used for low noise gears (JP-A-2-66365); and method of adding a thermoplastic elastomer and an addition product of alkylene oxide to alcohol to a polyacetal block copolymer used as polyoxymethylene resin. According to the methods known in these prior art references, the sliding characteristics at ambient temperature of the polyoxymethylene resins can only be improved, but these known methods are not effective at all to solve the problems caused by increasing the friction coefficient and wear resistance at high temperature.
Furthermore, there have been also known in prior art references that a method of blending a polyolefin to the polyoxymethylene resin (JP-B-42-19498); a method of adding a liquid form ethylene-.alpha.-olefin random copolymer to the polyoxymethylene resin (JP-A-4-239566); and a method of adding a modified .alpha.-olefin type polymer to the polyoxymethylene resin (JP-B-59-204652). It should be noted that the sliding characteristics at high temperature of conventional polyoxymethylene resin compositions cannot be improved at all by the methods known in these prior art references.